I generally start with a few mins on a recumbent bike to get the knees warmed up and heart rate up, then do some straight leg swings (front/front and side/side) to open up the hips somewhat. Then I start squatting sets of 5, starting with 2 sets on an empty bar, and then add 25's/45's until I'm at about 50% of my working weight. Once I get past 50% of my working weight I drop the reps to 3/2/1 reps or so. Usually I'm really stiff until I get to at least 135 or so.